interstellar maser

A maser formed through the interaction between
high-energy starlight and a nearby region rich in molecules; the first was
discovered in 1965. The conditions needed to produce an interstellar maser
tend to be found especially with star-forming regions and with late-type
stars that are losing mass.

The environs of stellar nurseries contain dense pockets of molecular material
rich in hydroxyl (OH), water (H2O), and methanol (CH3OH)
molecules. Nearby luminous infant O or B stars heat their dusty envelopes
with energetic photons, causing the dust to reemit in the infrared,
which in turn excites the molecules into maser action. Within a cloud the
maser-emitting region is no more than about 100 billion kilometers across
(about 10 times the size of Pluto's orbit).
Its power is given in terms of the number of maser photons emitted per second.
For typical H2O masers in star-forming regions, such as those
in the Orion molecular cloud, this is about 1046 s-1;
however, the H2O maser associated with W 49, a Wolf-Rayet
star, puts out 1049 s-1, making this the strongest
maser source in the Galaxy. The intensity of maser emission can vary dramatically
on a time scale of a year. One of the most spectacular examples is the H2O
outburst in Orion that happened in late 1979 and that lasted for 8 years.
The intensity of this one maser component suddenly increased by a factor
of 1,000, making it temporarily the brightest H2O maser source
in the sky.

Observational evidence indicates that once the young star begins to radiate
through nuclear fusion, OH masers survive only until the ionized region
expands to a diameter of about 0.3 light-year, while H2O masers
last for about 100,000 years after the star switches on. OH and H2O
masers have also been found in the nuclei of active galaxies, such as NGC
3079 and NGC 1068. The power of these extragalactic masers are stronger
(by about a million for OH masers (megamasers) and by a thousand for H2O
masers) than known maser sources in our galaxy.

Masers are also found near long-period
variables (LPVs), and arise when the turbulent upper photosphere of
a luminous star undergoing mass loss is exposed to the radiation from below.
These stars are typically class M (with 10,000 solar luminosities) and have
cool surface temperatures, usually around 2,500K. Since they are evolved,
their photospheres contain appreciable abundances of heavier elements, including
silicon and oxygen, which supply an ideal environment in which silicon dioxide
(SiO) masers can form. The LPV VY Canis Majoris is a prime example of such
a star. VY CMa shows a triple peaked SiO maser line at 43 GHz. Physically,
this has been interpreted as a spherically symmetric circumstellar envelope
with an inner maser region at rest relative to the star, and an outer masing
region expanding away from the star at around 10 km/s; this gives rise to
blue- and red-shifted peaks on either side of the one at rest. For more
details, see circumstellar maser.

Because interstellar maser emission often is very strong and arises from
extremely compact regions, it can be observed with interferometric methods,
such as very long baseline interferometry, that yield high spatial resolution.
Such observations provide detailed information on the physical conditions
in the emission regions and their chemical composition, velocity, and magnetic
field structure.

Molecules that exhibit maser action
in celestial objects

molecule

name

frequency (GHz)

characteristics*

OH

hydroxyl

1.612

O, M

"

1.667

O, M

"

1.720

O

H2CO

4.829

O

CH3OH

methanol

12.178

O

SiS

silicon sulfide

18.155

C

H2O

water

22.235

O, M

NH3

ammonia

23.870

O

SiO

silicon oxide

43.122

M, S, O

"

86.243

M, S

HCN

hydrogen cyanide

89.087

C

*O means that the maser emission is frequently found in star-forming regions;
M, in M stars; S, in S stars; C, in carbon stars